Yoku is putting on sunscreen. He uses $2\text{ ml}$ to cover $50\text{ cm}^2$ of his skin. He wants to know how many milliliters of sunscreen $(c)$ he needs to cover $325\text{ cm}^2$ of his skin. How many milliliters of sunscreen does Yoku need to cover $325 \text{ cm}^2$ of his skin?
Solution: We're dealing with a proportional relationship, so each ratio of milliliters to square centimeters must be equivalent. Let's use a table to see how many square centimeters of skin is covered with one milliliter of sunscreen. ${25}$ $\longrightarrow$ ${1}$ ${50}$ $\longrightarrow$ ${2}$ $ \times {\dfrac{1}{2}}$ $ \times {\dfrac{1}{2}}$ ${{cm}^{2}}$ $\longrightarrow$ ${{ml}}$ One milliliter of sunscreen covers ${25} \text{ cm}^2$ of Yoku's skin. Now, we can use this rate to solve for how many millimeters of sunscreen is needed to cover $325 \text{ cm}^2$ of skin. ${325}$ $\longrightarrow$ ${c}$ ${{25}}$ $\longrightarrow$ ${1}$ $ \times \dfrac{325}{{25}}$ $ \times \dfrac{325}{{25}}$ ${50}$ $\longrightarrow$ ${2}$ ${{cm}^{2}}$ $\longrightarrow$ ${{ml}}$ $c=1\times\dfrac{325}{{25}}=13$ Yoku needs $13 \text{ ml}$ of sunscreen to cover $325 \text{ cm}^2$ of his skin.